It is common practice in the processing of gold ores to use high pH cyanide solutions to dissolve the gold from the parent ore. This dissolution process forms a gold-cyanide complex. It is known that the presence of carbon, especially organic carbon, can greatly hinder the recovery of the precious metal from the ore.
Carbonaceous ore typically contains between 0.01 and 1.0 oz./ton of gold, usually associated with silver, and 0.25%-3% organic carbon by weight. The organic carbon is believed to impede the recovery of the precious metal by one or both of the following mechanisms:
1. The carbon in the ore absorbs or adsorbs the gold-cyanide complex formed by the dissolution of the gold by the alkaline cyanide solution. This renders the gold non-recoverable. In the absence of carbon in the ore, the gold complex would normally be recovered by any one of several processes and the gold later separated from the complex. PA1 2. The gold is believed to be intimately associated with the carbon in the ore. The gold is either adsorbed on active carbon elements and/or associated as organometallic or other organic chelate compounds which are not amenable to conventional cyanidation. Smaller portions of the total gold content may be present as clay-like mineral matrix. PA1 (a) Reactions of chlorine dioxide and organic carbon: The carbon compounds are believed to be similar in structure to humic acids such as those found in Leonardite. These humic acid extracts contain long chain carboxylic acids as well as sulfur and nitrogen groups. It is well known that the energy required to oxidize typical sulfur, nitrogen and carboxyl groups encountered in long chain organic materials is minimal. Hence, chemical oxidation such as chlorine dioxide can cause chain cleavage, oxidation of sulfur, nitrogen and carboxyl groups, and reaction at points of unsaturation. PA1 (b) Reaction of chlorine dioxide and gold: It is believed that a substantial amount of the gold is locked up in the form of a chelate containing CO-N-S ligands. Disruption of these ligands causes the ionic gold to be freed and solubilized, most likely in the form of a chloride and/or chloride complex.
Regardless of the predominate mechanism involved, it is common practice to pretreat carbonaceous ore containing precious metals with an oxidizing agent to destroy or reduce the effects of the carbon.
Over the years a variety of oxidizing agents have been proposed for the pretreatment of carbonaceous ores. These prior art techniques involved the use of ozone (U.S. Pat. No. 3,574,600), alkaline hypochlorite (U.S. Pat. No. 3,639,925), chlorine gas (U.S. Pat. No. 4,038,362), and oxygen (4,289,532).